Sera from patients undergoing hemodialysis with formaldehyde (F)-sterilized dialyzers were studied to determine if antibodies against F conjugated to human serum albumin (HSA) could be detected. F-human serum albumin (F-HSA) conjugates were prepared using ratios of F to HSA that did not precipitate the HSA. The F-HSA conjugates migrate differently electrophoretically than HSA with an increased negative charge of F-HSA as compared with HSA. The F-HSA was used in an ELISA. The results demonstrated that in certain sera, IgG, IgM, IgA and IgE antibodies against F-HSA could be measured. In the highest titered sera, it was shown that the IgG antibody was not directed against F alone or F-lysine but against an antigenic grouping of F-HSA. No correlation of either IgG or IgE antibodies with immune complex or allergic reactions was found in this series of dialysis patients. Some sera from dialysis patients had antibody activity against HSA. Sera from 2 physicians with rhinitis after F exposure had no antibody activity against F-HSA or HSA. Two nurses with a history of F-induced asthma had no IgG antibodies but did have IgE antibodies against F-HSA and HSA. This spectrum of immunologic responses is analogous to responses in dogs immunized with F or F dog albumin. We have not been able to identify anti HSA antibodies in patients reactive to other hapten-HSA compounds and it is suggested that anti HSA antibodies in F-exposed humans may relate to the F exposure.